1. Field of the Invention
The invention relates to a method of joining two cables with an insulation of cross-linked polyethylene or another cross-linked linear polymer.
2. The Prior Art
When joining two cables which have an insulation of cross-linked polyethylene, the insulation is initially removed nearest to the ends of the cables which are to be joined, usually in a fashion such that the insulation, located nearest the ends is bevelled in the direction of the ends. Then, before joining the ends together, usually achieved by welding or soldering, a semiconducting layer is applied around the joint (at least in the case of a high-voltage cable), and the joint, is normally insulated by wrapping the area around the exposed cable conductors and those parts of the conductor insulation which are located nearest the conductor ends with several layers of an unvulcanized polyethylene tape containing a cross-linking agent, for example di-.alpha.-kumyl peroxide or di-tert. butyl peroxide. The taped and joined cable is then placed with the wrapping in a tool where the wrapping is subjected to such a pressure and such a temperature that the layers of the tape melt together to form a cross-linked homogeneous compound. The tool used consists in the known embodiment of two tool halves which are each provided with a semicylindrical recess and which are movable towards each other. The tool halves contain built-in heating coils for heating them. At least if the cable is a high-voltage cable, a semiconducting layer is applied onto the insulation at the joint either after or in connection with the formation of this insulation. The same method can be used when joining cables which have insulation which consists of a cross-linked linear polymer other than polyethylene. In such cases the same linear polymer is normally used in unvulcanized form in the tape.
It has been found that when manufacturing the insulation at the joint in this manner, an insulation is obtained which does not possess uniform properties in the whole cross-section. Void formations will occur in the polyethylene material or in the material of other linear polymers, as well as flows of the polyethylene or of other linear polymers, and this may cause a deformation of the insulation at the joint so that the intended final shape is not obtained.